The purpose of the present work is to establish the function of IgD on murine lymphocytes. This will be done by determining the following properties of cells bearing IgD (1) Are IgD-only cells more readily triggered than tolerized? (2) Do IgD-only cells (as opposed to cells bearing only IgM or both IgM and IgD) give rise to cells secreting IgM or IgG, i.e., are IgD-bearing cells "memory" cells? In addition to probing the above questions we will determine whether the acquisition of IgD or IgM-bearing cells is independent of the presence of antigen and whether both isotypes on the same cell express the same idiotype.